Large portable liquid mixing tanks such as those used to handle automotive paint and solvents have a top with a large access opening that is closed and sealed by a so-called manhole cover for storage and transit. The manhole cover is removed when the paint is to be used and a liquid mixer with mounting cover is sealingly secured in its place to mix the paint or other liquid requiring mixing before use. In either of these closed and sealed conditions, the pressure of any air trapped in the tank when it is empty or partially full and in temporary storage or transit will rise with increasing ambient temperature. The portable automotive paint tanks can be as large as 575 gallon capacity and have a round access opening with a diameter as large as 22 inches and a pressure increase of just a few psi can develop a force on the large area of the manhole cover and liquid mixer mounting cover that can propel or blow it off when released from connection to the tank. The thus propelled object can possibly harm the person effecting its release and/or others in the vicinity in addition to allowing any paint or solvent then in the tank to be spewed out the tank opening. A bung may also be installed in the top of these tanks and it is also prone to being propelled as well if existing air pressure in the tank is not somehow quickly and efficiently relieved before it is freed from the tank.
There are of course a variety of manually operated pressure relief devices for liquid storage tanks, barrels, casks and the like such as those disclosed in U.S. Pat. Nos. 228,159 and 5,016,775, French Patent Numbers 16,806 and 1,142,713, and British Patent Number 1,525,969. However, these prior pressure relief devices are of limited use and not well suited to relieving the pressure in these large portable liquid mixing tanks quickly enough nor do they offer the opportunity of utilizing existing features associated with some if not most of these large portable liquid mixing tanks.